types and amounts of organic matter 4691) to determine the distribution of organic matter in...

16. ORGANIC FACIES OF CRETACEOUS AND JURASSIC SEDIMENTS FROM DEEP SEADRILLING PROJECT SITE 534 IN THE BLAKE-BAHAMA BASIN, WESTERN NORTH ATLANTIC1

Colin P. Summerhayes,2 Exxon Production Research Company, Houston, Texasand

Theodora C. Masran, c/o S. A. J. Pocock, Esso Resources Canada Limited, 339-Fiftieth Avenue South East,Calgary, Alberta, Canada T2G 2B3

ABSTRACT

The organic facies of Early and middle Cretaceous sediments drilled at DSDP Site 534 is dominated by terrestriallyderived plant remains and charcoal. Marine organic matter is mixed with the terrestrial components, but through muchof this period was diluted by the terrestrial material. The supply of terrestrial organic matter was high here because ofthe nearness of the shore and high runoff promoted by a humid temperate coastal climate. Reducing conditions favoredpreservation of both marine and terrestrial organic matter, the terrestrial materials having reached the site mostly in tur-bidity currents or in the slow-moving, near-bottom nepheloid layer. An increase in the abundance of terrestrial organicmatter occurred when the sea level dropped in the Valanginian and again in the Aptian-Albian, because rivers dumpedmore terrigenous elastics into the Basin and marine productivity was lower at these times than when sea level was high.

A model is proposed to explain the predominance of reducing conditions in the Valanginian-Aptian, of oxidizingconditions in the late Aptian, and of reducing conditions in the Albian-Cenomanian. The model involves influx ofoxygen-poor subsurface waters from the Pacific at times of high or rising sea level (Valanginian-Aptian, and Albian-Cenomanian) and restriction of that influx at times of low sea level (late Aptian). In the absence of a supply of oxygen-poor deep water, the bottom waters of the North Atlantic became oxidizing in the late Aptian, probably in response todevelopment of a Mediterranean type of circulation. The influx of nutrients from the Pacific led to an increase in pro-ductivity through time, accounting for an increase in the proportion of marine organic matter from the Valanginian intothe Aptian and from the Albian to the Cenomanian.

Conditions were dominantly oxidizing through the Middle Jurassic into the Berriasian, with temporary exceptionswhen bottom waters became reducing, as in the Callovian. Mostly terrestrial and some marine organic matter ac-cumulated during the Callovian reducing episode. When Jurassic bottom waters were oxidizing, only terrestrial organicmatter was buried in the sediments, in very small amounts.

INTRODUCTION

It is widely recognized that the types and amounts oforganic matter in marine sediments are responses to thesupply and preservation of organic material from ma-rine or terrestrial sources (Arthur, 1979; Welte et al.,1979; Tissot et al., 1980; Summerhayes, 1981). Exami-nation of the organic matter buried in marine sedimentscan tell us whether bottom waters were oxidizing or re-ducing (through good versus poor preservation of easilydecomposable materials); whether the surface waterswere productive or not (through the presence or absenceof organic enrichment); and whether the climate of thehinterland was humid or arid (through the relative abun-dance of marine and terrestrial organic matter).

We have examined the distribution of organic matterin sediments recovered at DSDP Site 534, in the Blake-Bahama Basin, with the following objectives in mind:

1) to determine the distribution of organic matter inMesozoic sediments at this site (for location, see Fig. 1);

2) to determine the organic facies of these sediments;3) to establish the probable controls on deposition of

organic matter here; and

Sheridan, R. E., Gradstein, F. M., et al., Init. Repts. DSDP, 76: Washington (U.S.Govt. Printing Office).

2 Present address: BP Research Center, Chertsey Road, Sunbury-on-Thames, MiddlesexTW 16 7LN, United Kingdom.

4) to relate our findings to the deposition of organicmatter in the Mesozoic at other sites in the westernNorth Atlantic (Fig. 1), and to draw inferences aboutpaleocirculation and climate.In many respects this study is a follow-up to earlierwork on the nature and controls of deposition of the or-ganic facies of the mid-Cretaceous "black shales" thatare widespread throughout the deep North Atlantic(Summerhayes, 1981). It is also a part of Exxon's ongo-ing program of studies on the nature and origin of or-ganic matter in deep-sea sediments (Mclver and Rogers,1978; Johnson et al., 1979; Gilbert et al., 1980; Gilbertand Summerhayes, 1981, in press; Summerhayes andGilbert, 1982, in press). This site is particularly interest-ing in that Jurassic "black shales" were recovered, alongwith the anticipated mid-Cretaceous ones. These are thefirst organic-rich Jurassic sediments recovered from deepwater by DSDP in the North Atlantic.

TYPES AND AMOUNTS OF ORGANIC MATTER

The drilled section consists of several different geo-logic units, each with a characteristic lithology (Table1). Shipboard geochemical reports by Sheridan, Grad-stein, et al. (Site 534 report, this volume) and postcruisestudies by Herbin et al. (this volume), confirm that thedifferent units tend to have different organic facies, asexplained in the next sections. Our data (Table 2) sup-plement the shipboard reports but are less comprehen-

469

C. P. SUMMERHAYES, T. C. MASRAN

40c

35C

75° 70°

Location of DSDP Site 534 and of other sites mentioned in text.

Table 1. Characteristic lithologies and organic geochemical character of Site 534 Mesozoicsection—based on shipboard reports.

Formation

Plantagenet

Hatteras

Blake-Bahama

Cat Gap

Unnamed

Subunit

4a

4b

4c

4d

5a

5b

5c

5d

6a6b7a7b7c

Sub-bottomdepth(m)

741-764

704-887

887-914

914-950

950-1098

1107-1202

1202-1268

1268-1342

1342-14281428-14961496-15491549-16171617-1635

Agea

MA

AL-CE

AP

BA-AP

HA-BA

VA

BE-VA

BE

TI-BEOX-TIOXCA-OXCA

Lithology

Red-brown variegatedclaystone

Black carbonaceousclaystone with greeninterbeds

Red-brown variegatedclaystone

Carbonaceous claystone

Carbonaceous claystoneLaminated chalk

Laminated chalk andclaystone

Laminated chalk andclaystone

Nonlaminated chalkand claystone

Claystone and limestoneClaystone and limestoneVariegated claystoneLimestone and claystoneGreen black and brown

claystone

Max. TOC(%)

0.7 b

(0.2)4.0b

(2.7)

1.8b

(0.5)4 .6 b

(2.8)4 .0 b

(2.3)(2.8)

(0.9)

(0.6)

(3.1)(0.3)(3.9)1.5b

(2.8)

Hydro;index

110

157

125

200

160

100

ND

ND

NDNDND

750

Pyrolysis

>en Organicc type

III

upper IIIlower II-III

III

II-III

II-III

III

ND

ND

NDNDNDResidualIII

a MA = Maestrichtian; CE = Cenomanian; AL = Albian; AP = Aptian; BA = Barremian, HA = Hautervian; VA =Valanginian; BE = Berriasian; TI = Tithonian; KI = Kimmeridgian; OX = Oxfordian; CA = Callovian.

" From shipboard organic geochemistry reports: other TOC data, in parentheses, are from Appendix I (this volume).c Average hydrogen index calculated from shipboard data presented in the Site 534 report, this volume. Organic type

estimated by evaluating average hydrogen indices against our Figure 2. ND = not determined.

470

ORGANIC FACIES OF CRETACEOUS AND JURASSIC SEDIMENTS

Table 2. Percent TOC, elemental ratios (H/C and O/C), pyrolysis data, and organic matter types.

Formation

Plantagenet

Hatteras

Blake-Bahama

CatGap

Unnamed

Subunit

4a

4b4b4b4b4c4d

5a5a5a5a5b5b5b5c5c5d5d

6a6a6a

7c

Sample(core-section

interval in cm)

24-3, 120-135

28-1, 140-15034-1, 132-14134-1, 132-14137-3, 120-13041-5, 140-15044-2, 120-130

48-4, 120-13150-2, 120-13256-1, 142-15059-2, 120-13066-3, 140-14972-2, 120-13572-2, 120-13577-2, 120-13083-4, 135-14089-4, 120-13389-4, 120-133

95-4, 138-15099-1, 115-130102-3, 130-140

125-5, 131-135

Sub-bottomdepth (m)

741.5

774.0831.0831.0859.5896.0923.0

959.0972.5

1026.51053.51116.51166.01166.01211.01260.51313.01313.0

1362.51395.51419.0

1612.5

Age

MA

CEALALALAPAP

BABAHAHAVAVAVAVABEBEBE

TIKIKI

CA

Lithology

Red variegated claystone

Laminated carbonaceous claystoneLaminated carbonaceous claystoneLaminated carbonaceous claystoneLaminated carbonaceous claystoneVariegated claystoneCarbonaceous claystone

Carbonaceous radiolarian micriteCalcareous claystoneCalcareous claystoneCalcareous siltstoneCalcareous mudstoneChalkClaystoneClaystoneLimestoneLight limestoneDark claystone

Red calcareous claystoneRed calcareous claystoneGreen calcareous claystone

Claystone

TOC

0.07

1.350.860.530.760.110.63

2.940.880.700.540.920.090.200.720.070.060.12

0.210.190.16

1.32

H/C

—

0.68——

0.53—

0.50

_

_—

——

——

1.13

O/C

—

0.16————

0.22

_

_——

——

——

0.16

ST

—

45252545—45

101010_20605035———

6060—

5

PS

—

155

—15—10

5

_5

_5

10———

_

——

5

Organic-matter type

C

—

20201525—30

101010_10353020———

4040—

5

AM

—

1015————

453545_30——10———

——

80

GA

—

—

5————

152515_15——————

_

——

—

SM

—

10306015—15

151010—10

51525———

—

——

5

RB

—

—

—————

155

10—10——————

—

——

—

HI

—

105132141111—36

4361001514479446069428375

196875

200

OI

-

237

106196207—600

348532465668457

23551760520

295713331966

102812681281

168

Note: Elemental analyses of extracted organic matter by Robertson Research (US) Inc.;TOC by LECO at Exxon Production Research Co. Organic-matter types by T. C. Masran(in percent of kerogen fraction) using organic matter classification scheme of Masran and Pocock (1981). ST = structured terrestrial, PS = pollen and spores, C = charcoal; AM= amorphous; GA = gray amorphous; SM = structured marine, RB = amorphous round bodies. Formations = Plantaganet, Hatteras, Blake-Bahama, Cat Gap, and Callo-vian unnamed. For age abbreviations see Table 1. HI = hydrogen index (mg hydrocarbons/g TOC), OI = oxygen index (mg Cθ2/g TOC), by Rock-Eval pyrolysis. — indicatesno available data.

sive, in that we analyzed only pieces of the cores frozenfor organic geochemical work. We analyzed these sam-ples by LECO for total organic carbon (TOC), by py-rolysis for hydrogen index and oxygen index, and vis-ually for different types of organic matter; some sam-ples were analyzed elementally for the C, H, and O con-tents of kerogen extracts (Table 2).

Plantagenet Formation

The brownish colored claystone of Subunit 4a con-tains very little organic matter (Tables 1 and 2). Ship-board analysis by pyrolysis shows that the organic mat-ter from this unit has a low hydrogen index typical of

kerogen classified as Type III and probably having a ter-restrial source (Table 1, Fig. 2; and Site 534 report, thisvolume). There was too little particulate organic matterin our sample to permit detailed analysis of kerogentype. The red brown coloration and lack of organic mat-ter suggest deposition under oxidizing conditions (Site534 report, this volume).

Hatteras Formation

The Hatteras Formation is characterized by abun-dant carbonaceous claystone and scarcity of carbonate(Table 1; and Site 534 report, this volume). This forma-tion contains the main "black shale" units of the North

2 00

1.50

1.00

0.50

Ai

I- w

i i •

Hf

^ •ft——

Residual 0. M.

1 , 1 ,

1

—

I l l

1000

800 -

600 ~

•= 400 ÷

200

0.10 0.20 0.30

O/C

100 200 300 400

Oxygen index (mg CO2/g TOC)

Figure 2. Characterization of organic matter (O.M.) into four main types (I—III and residual) by A.elemental analysis and B. pyrolysis, based on Tissot et al. (1980).

471


Atlantic. Sheridan, Gradstein, et al., (Site 534 report,this volume) divide it into three parts (Subunits 4b, 4c,and 4d) based on lithology (Table 1). All of the subunitsare claystones, because deposition took place in deepwater at a time when the carbonate compensation depth(CCD) was high in the water column (Thierstein, 1979).

Subunits 4b and 4d are characterized, as is much ofthe mid-Cretaceous "black shale" of the deep NorthAtlantic, by alternating interbeds of black carbonaceousclaystone relatively rich in organic matter and greenishclaystone poor in organic matter (Site 534 report, thisvolume). The contrast between these adjacent layers canbe seen from Table 2 (Core 34, Section 1): the blacklayer contains about twice as much organic matter as thegreen one. DSDP data files show that in these unitsTOC fluctuates from 0.1 to 2.8%, and we calculatefrom those data that the average TOC of the rich units(with more than 1% TOC) is 1.7%. Shipboard datashow that TO Cs reach 4.0 to 4.6% in these two units(Table 1) (Site 534 report, this volume). Our richest sam-ple contained 1.35% TOC (Table 2—Core 28, Section1); that of Herbin et al. (this volume) had 2.6% TOC.

Most of the carbonaceous layers are finely laminated,whereas many of the intervening green ones are biotur-bated (Site 534 report, this volume). This change in sedi-mentary structure is taken to reflect changes in the de-gree of oxygenation of the depositional environment,the laminations representing periods when bottom wa-ters were so low in oxygen as to prevent much benthicfaunal activity (we will refer to these conditions as re-ducing).

Oxidizing conditions prevailed throughout the depo-sition of Subunit 4c, which is a red brown, variegated,and bioturbated claystone containing very little organicmatter (Tables 1 and 2; and Site 534 report, this vol-ume).

Our five samples from the carbonaceous units of theHatteras Formation (Subunits 4b and 4d) contain an or-ganic facies dominated by terrestrially derived materi-al (including charcoal), with subordinate amounts ofstructured marine organic matter and almost no amor-phous material (Table 2). One sample in Core 34, Sec-tion 1 has a dominantly marine organic facies. The aver-age organic facies for the five samples is 37% structuredterrestrial organic matter, 9% pollen and spores, 22%charcoal, 6% amorphous, and 26% structured marineorganic matter. There was too little organic matter pres-ent for us to be able to characterize the organic facies ofthe variegated claystone (Subunit 4c).

Organic matter may also be characterized by ele-mental analysis and by pyrolysis into four main types oforganic matter, as shown in Figure 2. Shipboard pyroly-sis measurements (Site 534 report, this volume) show adownhole increase in the hydrogen index of kerogenfrom Subunits 4b to 4d, interrupted by a zone of low hy-drogen index corresponding to Subunit 4c (Table 1).The hydrogen indices tend to follow the TOC (Table 1),as seen at other nearby sites in the western North Atlan-tic (Fig. 3). This pattern is interpreted as showing thatthe TOC increases in proportion to the preservation oflabile, hydrogen-rich components, most of which are

450

400

350

ü

° 300

250

200

150

100

50

y

o D

>/ °Site

CE-AL

AL-AP

418

•

O

417

•D

CE = CenomanianAL = AlbianAP = Aptian

5

TOC(%)

10

Figure 3. Dependence of TOC on preservation of hydrogen-rich or-ganic matter. (Based on data from Deroo et al. [1980]. See Fig. 2for limits to organic matter Types II and III.)

amorphous. Although we do not see a well-defined rela-tionship between TOC and amorphous content at Site534 (Table 2), such a relationship is characteristic ofboth the eastern and western North Atlantic Cretaceousblack shales (Summerhayes, 1981).

Consideration of the average hydrogen index alone(Table 1) gives the impression that the organic matter ofSubunits 4b and 4d belongs to Type III, which is usuallythought of as terrestrially derived (Fig. 2). Actually, al-though many samples do belong to Type III, some be-long to the residual category (of Fig. 2) and consist ofhighly refractory material—probably charcoal (e.g.,Core 44, Section 2—Table 2), whereas others belong toType II and are probably derived mainly from marineorganic matter (Site 534 report, this volume). Thus weare dealing with a mixed marine-terrestrial organicfacies that is dominated by terrestrial-plant remainsmixed with charcoal. The downhole increase in averagehydrogen index from Subunits 4b to 4d (Table 1) prob-ably reflects an increasing admixture of marine materialin the older samples. This increasing admixture goesalong with an increase in the "marine" character of theclay mineral assemblage (Site 534 report, this volume).Herbin et al. (this volume) agree that Subunit 4b has lowhydrogen indices, and they show from gas chromatog-raphy that the organic extract is highly terrestrial incharacter. Our pyrolysis data confirm the Type-Ill orresidual character of the organic matter of the HatterasFormation (Table 2). The association of a marine or-ganic facies with a low hydrogen index in Core 34, Sec-tion 1 (green sediment with low TOC, probably depos-ited under oxidizing conditions) suggests that not allType-Ill organic matter is necessarily terrestrial. It maybe biologically degraded marine organic matter that haslost its hydrocarbon-generating potential. We would notexpect to see this degradation in samples that were de-posited under reducing conditions.

472


These findings confirm earlier reports that the or-ganic facies of the mid-Cretaceous "black shales" in thewestern North Atlantic is dominated by terrestrial or-ganic matter (Tissot et al., 1980; Summerhayes, 1981).At some other western North Atlantic DSDP sites (e.g.,Site 105), parts of the Cenomanian are unusually rich inmarine organic matter (Tucholke and Vogt, 1979; Sum-merhayes, 1981). Sheridan, Gradstein, et al. (Site 534report, this volume) thought that they could see someevidence for this Cenomanian "abundance spike" atSite 534, but we could find no evidence for either TOCenrichment or concentrations of marine organic matterin Cenomanian samples at Site 534; the maximum TOCvalue for the Cenomanian samples analyzed by DSDP is2%, and the pyrolysis data indicate Type-Ill organicmatter. If the richly marine Cenomanian interval wasdeposited at this site, it may have been eroded.

Blake-Bahama Formation

This formation is highly calcareous, dominated bychalks and interbedded calcareous claystones, some ofwhich are carbonaceous. The most carbonaceous bedsoccur toward the top of Subunit 5a, which is transi-tional to the Hatteras Formation (Table 1; and Site 534report, this volume). Much of the Blake-Bahama For-mation is turbiditic, implying the lateral introduction oforganic matter. Deposition took place at a time whenthe CCD was much deeper than it was in the Barre-mian-Cenomanian (Thierstein, 1979; Site 534 report,this volume). Many of the chalks and claystones arelaminated, especially in Subunits 5a, 5b, and 5c, sug-gesting that bottom waters were poorly oxygenated;Subunit 5d is not laminated, suggesting that bottom wa-ters were oxidizing at the beginning of the Early Cre-taceous (Site 534 report, this volume).

The calcareous claystones tend to have more organicmatter than the chalks, as seen, for instance, in Core 72,Section 2 and Core 89, Section 4 (Table 2). As men-tioned above, the upper part of Subunit 5a has the mostorganic matter. Subunit 5b is also enriched in organicmatter, Subunit 5c is less so, and Subunit 5d not at all,as shown also by Herbin et al. (this volume). Back-ground TOCs are about 0.1 % (Table 2; Site 534 reportand Appendix I, this volume), values typical of pelagicdeep-sea sediments (Mclver, 1975).

Analyses of organic matter type show that there aretwo organic facies within the Blake-Bahama Formation(Table 2). One, in samples with more than 0.5% TOC, isdominated by amorphous organic matter: it averages55% amorphous material, 12% structured marine ma-terial, 17% structured terrestrial material, 4% pollenand spores, and 12% charcoal. The other, in sampleswith less than 0.5% TOC, is much richer in refractorycomponents, averaging 55% structured terrestrial ma-terial, 2.5% pollen and spores, 32.5% charcoal, and10% structured marine material. This facies change ischaracteristic of the Early and mid-Cretaceous sedi-ments of the deep western North Atlantic (Summer-hayes, 1981). It is attributed to decomposition of labileamorphous and marine organic matter during periods

when bottom waters were oxidizing, and preservation ofthose materials when bottom waters were reducing.

Pyrolysis data show that the bulk of the organic mat-ter belongs to Type III (Tables 1 and 2, and Fig. 2); in-spection of Table 2 suggests that degraded marine ma-terial contributed to the terrestrial organic matter pro-viding the low hydrogen index. Some samples have highhydrogen indices, belong to Type II, and probably con-tain mainly marine-derived organic matter (Site 534report, this volume), especially in Subunit 5a (Core 48,Section 4, Table 2). Herbin et al. (this volume) recordedhydrogen indices up to 263 in Subunit 5a, up to about150 in Subunit 5b, and 0 in Subunit 5d (Subunit 5c wasnot analyzed). Our data confirm that low hydrogen in-dices characterize Subunits 5b, 5c, and 5d (Table 2).From the low average hydrogen index (Tables 1 and 2)and the moderately high amorphous content (Table 2),we deduce that some of the amorphous material in thesesamples may be derived from terrestrial plant remainsor is very highly degraded marine organic matter. Muchthe same conclusion was reached by Summerhayes(1981) and Masran (personal communication, 1978)about the amorphous organic matter in pre-Cenoman-ian Early and mid-Cretaceous sediments from elsewherein the deep western North Atlantic. Gas chromatog-raphy on samples from Subunit 5b supports a pre-dominantly terrestrial source for the organic matter(Herbin et al., this volume).

Cat Gap Formation

These Jurassic sediments are pinkish limestones andgreenish gray calcareous claystones that generally areneither laminated nor bioturbated (Table 1; and Site 534report, this volume). Local accumulations of carbon-aceous claystone suggest that bottom waters were anox-ic at times (e.g., in Cores 99-101, where TOCs reach amaximum of 3.1%—Table 1, and Site 534 report, thisvolume). Most of the sediments in the Cat Gap Forma-tion contain 0.1 to 0.3% TOC (Site 534 report, thisvolume; Herbin et al., this volume; and Tables 1 and 2),like typical pelagic sediments (Mclver, 1975). Organicmatter types are entirely refractory and terrestrial whereTOCs are low (Table 2), as is typical in highly oxidizingdepositional environments. Hydrogen indices are alsovery low (Table 2; and Herbin et al., this volume).

Callovian-Oxfordian SedimentsMost of the Callovian-Oxfordian section consists of

green and brown claystone, with some grey to black car-bonaceous "black shale" beds (Site 534 report, this vol-ume). The organic-rich beds are set in a "background"of organic-poor green claystones, or marly limestones.Limestone abundance decreases with depth as "blackshale" abundance increases until, in Core 125, "blackshale" makes up half of the core.

The carbonaceous beds, like that in Core 125, arelaminated and contain glauconite, phosphorite, fish-bones, and unusual benthic foraminiferal faunas, sug-gesting deposition in poorly oxygenated bottom waters(Site 534 report, this volume). TOCs reach 2.8% in Core

473


125 (Subunit 7c), but there is also organic-rich sedimentin Subunit 7a, where TOCs reach 3.1% (Table 1). Her-bin et al. (this volume) report TOCs as high as 3.1% inSubunit 7c, Core 126. The "background" sedimentsmostly contain 0.1 to 0.3% TOC, increasing in Subunit7c to about 0.6%; in contrast, shipboard analyses sug-gest that "background" TOCs in Subunit 7c may beabout 1.3%. Our analyses of a frozen sample from Core125 gave a TOC of 1.32% (Table 2); Herbin et al. (thisvolume) report 1.8% TOC in this core.

The organic facies of the sample that we analyzed(Core 125, Section 5) is dominantly amorphous withsubordinate terrestrial material and a little structuredmarine organic matter (Table 2). Shipboard pyrolysisdata suggest that organic-rich sediments in this samecore have hydrogen indices around 50, which is eitherlow Type III or residual organic matter (see Fig. 2). Ourelemental analyses suggest, in contrast, that the amor-phous material in Core 125 is marine-derived (Table 2,Fig. 2). Our pyrolysis data put this sample on the borderbetween Types II and III, suggesting that it is a mixtureof marine and terrestrial organic matter (Table 2, Fig.2). Herbin et al. (this volume) show that the TOC-richsamples have the higher hydrogen indices, which reach221 in Core 26, and 161 in Core 125, indicating an ad-mixture of terrestrial and aquatic organic material. Weconclude that the organic matter in these mid-Jurassicsediments is most probably dominantly terrestrial withsome marine admixture becoming important whereTOCs are high.

DISCUSSION

In the foregoing section we have confirmed that atSite 534 organic matter is concentrated in the mid-Cre-taceous, is present in lesser amounts in the Early Creta-ceous, occurs in some organic-rich beds in the Jurassic,and is not abundant in the Late Cretaceous. Most of theorganic matter is terrestrially derived. Marine organicmatter is not commonly abundant.

Our other objectives were to establish the controls ofdeposition of organic matter at this site, and to relateour findings to the history of deposition of organic mat-ter in the Mesozoic of the western North Atlantic inorder to enable us to draw inferences about the circula-tion of the basin and the climate of those times.

Distribution of Organic Matter through Time

In the Cretaceous sediments of the western NorthAtlantic there is a gradual increase in TOC enrichmentfrom the Berriasian to the Cenomanian. MaximumTOCs are 1 to 3 % in the Earliest Cretaceous (Berriasianto Valanginian); rise to 3 to 5% in the Hauterivian-Bar-remian-Aptian-Albian; and reach 10% or more in theCenomanian-Turonian (Thierstein, 1979, Fig. 12). Thereis a somewhat similar picture in the eastern North At-lantic (Thierstein, 1979, Fig. 13).

These changes in TOC with time may reflect changesin the rates of sedimentation of lithogenous or carbon-ate components, as well as changes in the rate of supplyor rate of preservation of organic matter. In particular,the enrichment of Barremian-Aptian sediments in TOC

could have been caused in part by the abrupt raising ofthe CCD from 5 km to 3 km at the Barremian/Aptianboundary (Thierstein, 1979). This event, which deline-ates the boundary between the calcareous Blake-Baha-ma Formation and the siliceous Hatteras Formation (Site534 report, this volume), can be seen in Figure 4A as asharp drop in the rate of accumulation of carbonate be-tween Subunits 5a and 4d. Clearly, carbonate deposi-tion must have obscured the TOC accumulation pictureby dilution in the Valanginian through the Barremian(Subunits 5c-5a—Fig. 4A).

The pattern of peak and background rates of accu-mulation of TOC at Site 534 is one of a steady increasefrom the Valanginian through the Barremian (Subunits5c through 4d) (Fig. 4B and 4C). As explained in thecaption to Figure 4, peak values represent accumulationunder reducing conditions, whereas background valuesrepresent accumulation under oxidizing conditions.Both the peak and background rates of TOC accumula-tion drop in the Aptian (Subunit 4c), when the oceanwas predominantly oxidizing, and rise again in the Albi-an-Cenomanian (Subunit 4d), when conditions were,again, predominantly reducing, with temporary inter-vals of oxidation (Fig. 4B, 4C).

Although the accumulation of TOC is independent ofthe accumulation of carbonate, it does show an inverserelation to the accumulation of lithogenous clastic com-ponents (Figs. 4 and 5). At times when rates of clasticinput were highest (Subunits 4c, 5c), rates of accumula-tion of organic matter were lowest, and vice versa. Thisinverse relationship is not caused by dilution of organicmatter by clastic input. Instead, it means that these twoindependent variables are responding in opposite waysto some outside force—in this case most probably achange in sea level. As sea level rises, clastic input wanesand either productivity increases or the rate of preserva-tion of organic matter increases. Because all sedimentsare laminated (except in Subunit 4c) and because therate of TOC accumulation changed, not only under re-ducing conditions (peak values, Fig. 4B) but also underoxidizing conditions (background values, Fig. 4C), itseems unlikely that there was a change in rate of preser-vation with time. A change in productivity with timeseems more probable: it would have required a changein the rate of supply of nutrients to the surface waters.We show later on in this report what may have causedthis rate to change.

By removing the carbonate component (calculatingTOCs on a Carbonate-free basis) we can improve ourview of the change in TOC with time in the westernNorth Atlantic. At DSDP Sites 105, 387, 391, and 534there is a steady increase in maximum Carbonate-freeTOC values from the Berriasian through the Valang-inian towards the Hauterivian (Fig. 6). Barremian sedi-ments have high TOC maxima, but there is a pro-nounced drop in TOC at all of these sites in the Ap-tian-Albian (Fig. 6); this drop can also be seen at Site417 (Fig. 6; and Hochuli and Kelts, 1980). At Site 534,this event is marked by deposition of red brown vari-egated clays under predominantly oxidizing conditions(Subunit 4c; Table 1). TOCs are high again in the Ceno-

474


5c h

CaCO3 (mg/cm /yr.) TOC peak average2

(mg/cm /yr.) x 100

TOC background average2

(mg/cm /yr.) x 100

2Clastic (mg/cm /yr.)

Figure 4. Rates of accumulation of carbonate, organic matter, and lithogenous clastic materials. (Calculated by using sedimentation rates of10 m/m.y. for Unit 4 [Subunits 4b-d], and 21 m/m.y. for Unit 5 [Subunits 5a-c] [Site 534 report, this volume], an average porosity of 30%,and an average density of 2.65 g/cm3. These give bulk accumulation rates of 1.86 mg/cm2/yr. for Unit 4, and 3.9 mg/cm2/yr. for Unit 5. TOCand carbonate data were averaged for each subunit; noncarbonate data include some biogenic opal. TOC accumulation rates are bimodallydistributed, with one population above 1 mg/cm2/yr. × 100, and another below that value. All of the smaller values were averaged to producea "background" TOC accumulation rate; the larger values were averaged to produce a "peak" TOC accumulation rate. Peak values representdeposition under reducing conditions; background values represent deposition under oxidizing conditions.)

I- × 4σ> •.

2 >

2 h

2.0 1.0

Clastic (mg/cvnr/yr.)

\

\

4d ^

5b\

•

\

5a

\5c

\

\

-

4b

^ 4 c

1.5

Figure 5. Interdependence of rates of accumulation of organic and lithogenous componentsdata from Fig. 4).

2.0

(based on

manian at Sites 105, 386, 387, and 417, but not at Sites391 or 534 (perhaps because of erosion?) (Fig. 6).

Relation of TOC to Changes in Sea Level

As implied earlier, the TOC pattern of the mid-Cretaceous may be linked to changes in sea level (Sum-merhayes, 1981; Scholle and Arthur, 1980). Vail et al.(1977) show that the sea level was high through theLatest Jurassic and into the Berriasian, dropped at theend of Berriasian times, then rose to a peak in theCenomanian, with a major interrruption in the late Ap-tian (Fig. 7). The two major sea-level lows (Valanginian,

corresponding to Subunit 5c, and Aptian, correspond-ing to Subunit 4c) are the times of highest rates of clasticsedimentation and lowest rates of accumulation of or-ganic matter (Figs. 4 and 7).

There is also a close relation between sea-level changeand the oxidation of bottom water in the mid-Cre-taceous North Atlantic. Sediments first became com-monly laminated in the Valanginian (Subunit 5c) (Site534 report, this volume) as sea level was rising (Fig. 7).We assume that the North Atlantic must have been wellstratified at this time, with predominantly reducing bot-tom waters, a state that continued into the Aptian.

475


ED

epth

I

8 0 0 -

9 0 0 -

1 0 0 0 -

1 1 0 0 -

1 2 0 0 -

1 3 0 0 -

Site 534

* 1,#* t

y-•—Oxic layer

•

• *•1

• * y'

ii

/

1 i ' i

Site 391CE

AL

; AP

BA

8 0 0 -

-? 9 0 0 -

1100-

CE

ZCE-ALAL

~AP-BA3BA-HA

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ep

th (

m)

Q

700-

800-

900-

1

; • -

• %i••

i i > i i i ' i

0 2

Site 417

4 6TOC(%)

CE J

" C E - A L •B 3 0 0 -

α.AL Q

BA

HA

VA

CE

A L

CEAL

"AP-AL

0 2 4 6 8 10TOC (%)

CE = Cenomanian; AL = Albian; AP = Aptian;BA = Barremian; HA = Hauterivian;VA = Valanginian; BE = Berriasian; TI = Tithonian

Figure 6. Downhole distribution of Carbonate-free TOC at DSDP sites. (Calculated from data given in ap-propriate DSDP site reports and Appendix I; data from Site 534 excludes two analytically oddball val-ues in Core 78, Section 4 and Core 79, Section 4.)

Short-term, periodic interruptions, probably climatical-ly induced and recurring at intervals of 10 to 50,000 yr.,made the bottom waters temporarily oxidizing (Arthurand Natland, 1979). When the bottom waters were ox-idizing, organic matter was consumed by benthic or-ganisms, reducing TOCs to "background" levels of lessthan 1% (Fig. 6).

The late Aptian sea-level drop (Fig. 7) is associatedwith widespread oxidation that persisted for a few mil-lion years and gave rise to the red brown sediments ofSubunit 4c at Site 534 (Figs. 4 and 6). Subsequently, asthe sea level rose (Fig. 7), conditions became again pre-dominantly reducing, enabling organic matter to accu-mulate in substantial amounts (Figs. 4 and 6). The oxi-dation event is seen in the TOC record at several of thewestern North Atlantic sites (Fig. 6), as mentioned ear-lier.

It is significant that the pattern of TOC accumulationis not simply one of constant flux diluted by clastic in-put. Oxidation affects accumulation, especially in di-minishing the TOC flux in the Aptian (Subunit 4c—Fig.4C, 4B). Also, as mentioned earlier, the gradual in-crease in both peak and background rates of accumula-

tion of TOC through time (Subunits 5c to 4d—Fig. 4C,4B) suggests that the North Atlantic became graduallymore productive as the sea level rose in the Valanginianthrough the Aptian.

Distribution of Organic Facies

The proportions of marine and terrestrial organicmatter changed with time during the Cretaceous. For in-stance, at Site 534, in samples with more than 0.5%TOC, there is more terrestrial organic matter in Sub-units 4b, 4c, and 5b, representing times of low or risingsea level, than in Subunits 4d and 5a, when sea level washigh (compare Tables 1 and 2 with Fig. 7). This changegoes along with a change in clay mineralogy (Site 534 re-port, this volume). If we assume that the flux of ter-restrial organic matter to the sediment parallels that ofelastics (Fig. 4D), then it is evident that much of the lowTOC flux at times of high clastic rates of accumulationshould be terrestrial (Fig. 4B, 4C), and that the flux ofterrestrial material should decline along with a decline inclastic input.

The proportions of marine and terrestrial organicmatter changed from one part of the North Atlantic to

476


70H

80 H

90 H

100

noH

120H

130 H

Figure 7. Relative changes of sea level in the Cretaceous (from Vail etal., 1977). (Tu = Turonian; see Fig. 6 for key to rest of Ages).

another. For example, in the western North Atlanticthere is much more terrestrial organic matter than in thesoutheastern North Atlantic in the mid-Cretaceous.This is thought to reflect the influence of climate andcirculation: the southeastern North Atlantic had an aridhinterland, low runoff, coastal upwelling, and high ma-rine productivity, whereas the western North Atlantichad a humid climate and high runoff, lacked coastalupwelling, and had lower productivity (Tissot et al.,1980; Summerhayes, 1981).

The organic facies also changed in relation to dis-tance from shore. For instance, in the western NorthAtlantic (except in the Cenomanian) we find more ter-restrial organic matter nearshore (e.g., DSDP Sites 105,391, and 534) than offshore (e.g., DSDP Sites 386, 387,417, and 418) (Summerhayes, 1981). This difference re-flects sedimentary processes, less terrigenous sediment(i.e., less terrestrial organic matter) being transported tothe offshore than to the nearshore sites to dilute themarine contribution. We calculate that dilution by elas-tics nearshore may be five times what it is offshore. Forinstance, the rate of accumulation of elastics offshore atSite 417 is about 0.3 mg/cm2/yr., a factor of five lessthan it is at Site 534 (see Fig. 4D). As a result, TOCs aremuch higher at Site 417 (averaging 4.7% in the Albian-Cenomanian, according to data from Deroo et al.,1980) than they are at Site 534 (see Tables 1 and 2).Rates of accumulation of TOC at both sites are aboutthe same (2.2 mg/cm2/yr. × 100 in the Albian-Ceno-manian at Site 417, 3.4 mg/cm2/yr. × 100 in the basalAptian-Albian at Site 417, and comparable values atSite 534, as seen from Fig. 4B). The organic facies atSite 417 is correspondingly less terrestrial and more ma-rine, on average, that at Site 534 (compare Table 1 withFig. 3). Site 417 has more Type II—III mixtures than

does Site 534. Nevertheless, the terrestrial influx is stilllarge enough to keep the hydrogen index below pureType-II material even at Site 417 (compare Fig. 2 withFig. 3).

At Site 417, because individual organic-rich beds maybe dominated by single dinoflagellate species, Hochuliand Kelts (1980) deduce that the source of the marineorganic matter most probably was periodic planktonblooms. The terrestrial organic matter was most prob-ably deposited both nearshore and offshore from sus-pension either in turbidity currents or in the slow-mov-ing, near-bottom nepheloid layer (Summerhayes, 1981;Site 534 report, this volume). We find no evidence tosupport the suggestion by Montadert, Roberts, et al.(1979), and Jenkyns (1980), that the high TOCs and ter-restrial character of the organic facies of the westernNorth Atlantic result from the influx of massiveamounts of terrestrial organic matter. Terrestrial organ-ic matter is always being supplied to the deep ocean bybottom currents but usually is oxidized during transpor-tation and deposition. The modern analog for the mid-Cretaceous western North Atlantic is the Black Sea,where organic-rich sediments desposited under reducingconditions contain large amounts of terrestrially derivedorganic components (Simoneit, 1977; Debyser et al.,1977).

Carbon Isotope RecordThe pattern of TOC distribution in the North At-

lantic has been linked to the isotopic geochemistry ofcarbon by Scholle and Arthur (1980). They note thatfrom the Berriasian into the Aptian-Albian there was asteady shift in δ13C toward more positive values inwhole-rock samples of pelagic limestone (Fig. 8). Theratios drop in the Albian and rise again in the Cenoman-ian (Fig. 8). Because the tests of carbonate organismsare in isotopic equilibrium with seawater, Scholle andArthur (1980) interpret their data to suggest that the sur-face waters of the North Atlantic gradually became iso-topically heavier from the Berriasian through to the Ap-tian-Albian. They argue that this isotopic change wascaused by the burial of marine organic matter (which isenriched relative to surface waters in 12C) beneath thereducing bottom waters of the Atlantic. Continuedburial of this organic matter would lead to enrichmentof surface waters in the heavier isotope 13C. If this inter-pretation is correct, it is added evidence of the presenceof reducing conditions throughout the Early Cretaceous(i.e., post-Berriasian into the Aptian).

It may be significant that there is in the isotopic dataa negative δ13C shift of short duration at the Aptian/Al-bian boundary (Fig. 8). Conceivably, this represents theperiod of oxidation during which the sediments of Sub-unit 4c were deposited. Overturn of the water columnand oxidation of organic matter at the bottom by ben-thic organisms would be expected to interrupt the posi-tive δ13C shift typical of the Earlier Cretaceous. Thenegative isotopic shift of the Albian-Cenomanian (Fig.8) occurred at a time when bottom waters were reducingbut when TOCs were not exceptionally high (Fig. 8). Incontrast, the Cenomanian event of basinwide enrich-

477


65

7 5 -

8 5 -

95-1

1 0 5 -

115-

1 2 5 -

135-

Stage

Maestrichtian

Campanian

Santonian

Coniacian

Turonian

Cenomanian

Aptian

Barremian

Hauterivan

Valanginian

Berriasian

Composite δ 1 3 Cwhole-rock

values

-3 -2 -1 / ^ + 1 +2 +3 +4

δ1 3C (PDB)

Pacific Tethys

Figure 8. Carbon isotopic variation through the Cretaceous (fromScholle and Arthur, 1980).

ment in marine organic matter (Fig. 8; Summerhayes,1981) may correlate with the positive δ13C shift near theCenomanian/Turonian boundary (Fig. 8; Scholle andArthur, 1980). One problem in trying to tie the isotopicdata and the TOC data together is the poor resolution ofthe biostratigraphic data in carbonate-depleted middleCretaceous sediments.

Jurrassic Organic Enrichment

The Jurassic organic-rich claystones of the Callovian(Core 125) developed because bottom waters in the earlyNorth Atlantic became temporarily poorly oxygenatedor reducing (Site 534 report, this volume). Depositiontook place most probably from suspension in a neph-eloid layer moved by weak bottom currents (Site 534report, this volume). The deposit is dominated by terres-trial organic matter with a subordinate marine compo-nent that is most abundant where TOCs are highest.

Oceanographic Model for the Mesozoic North Atlantic

The preceding discussions suggest that the Meso-zoic North Atlantic experienced a complex circulationhistory that we have tried to depict in a simple and sys-tematic fashion in Figure 9. This model does not take in-to account the short-term fluctuations between reducingand oxidizing conditions that occurred at intervals of10,000 to 50,000 yr. and gave rise to the white gray orblack green couplets typical of the Early and middleCretaceous. It developed from the suggestion of Thier-

Bahamas

C i I I

I /\ y

Bahamas

Bahamas

Jurassic—BerriasianSea level highNo Pacific connectionNegative water balancePredominantly oxidizing

Valanginian-Aptian

Sea level risingGood Pacific connectionPositive water balance (stratified)Predominantly reducing

Aptian—Albian

Sea level lowPoor Pacific connectionNegative water balancePredominantly oxidizing

Albian-CenomanianSea level highGood Pacific connectionPositive water balance (stratified)Predominantly reducing

Cenomanian EventOpening of South Atlantic

Introduces saline intermediate

waterRecharges surface with nutrientsProductivity high; bottom

water reducing

Latest CretaceousNo physical barriers to deep

circulationPredominantly oxidizing

Figure 9. A-F. Simplistic schematic model depicting the relation be-tween sea level, circulation, and development of oxidizing versusreducing bottom waters in the North Atlantic.

stein and Berger (1978) that the North Atlantic is "estu-arine" with respect to the Pacific, oxygen-poor andnutrient-rich subsurface water from the Pacific beingdrawn into the Atlantic at intermediate depths beneathless dense surface water. Today's Gulf of California,Baltic Sea, and Black Sea are "estuarine analogs" forthe Early Cretaceous North Atlantic. Movement of Pa-cific subsurface intermediate water from west to eastmay have taken place much as today in an ancestral Pa-cific Equatorial Undercurrent. If the movement of thissubsurface water had been reduced or shut off becauseof tectonic movements or by changes in sea level, circu-lation would have changed in the North Atlantic. We en-visage that the connection between the Pacific andNorth Atlantic was poor or nonexistent in Jurassic

478


through Berriasian times, during which a circulationperhaps like that of today's Mediterranean kept theBasin more or less well oxygenated (with minor excep-tions, as in the Callovian) (Fig. 9A). Once the Pacificconnection was established in the Early Cretaceous, asteady supply of oxygen-poor, nutrient-rich Pacificsubsurface water kept North Atlantic bottom waterswell stratified and poorly oxygenated (Fig. 9B) as thesea level rose (Fig. 7). We suggest that the Aptian-Al-bian sea-level drop (Fig. 7) cut off or reduced this sup-ply, making the North Atlantic less well stratified andpermitting a return to a negative water balance, prob-ably like that of today's Mediterranean, which oxy-genated the bottom waters (see Fig. 9C) (see Demaisonand Moore, 1980, for a discussion of negative versuspositive water balance). As the sea level rose, in the Al-bian, we surmise that the Pacific connection was rees-tablished or reinforced and that the North Atlantic be-came once more stratified and reducing (Fig. 9D). To-ward the end of the Cenomanian the opening of theequatorial Atlantic allowed saline water to enter atintermediate depths from the South Atlantic (Fig. 9E);this may have caused increased advection of nutrients tothe surface, leading to a basin-wide productivity event(Tucholke and Vogt, 1979; Summerhayes, 1981). An al-ternative explanation for the development of an organ-ic-rich deposit of marine organic matter in the Ceno-manian is that clastic dilution was very low then becauseof the high sea levels of that time (Fig. 7). The Early andmiddle Cretaceous North Atlantic Basin was disposed tobecome reducing, not only because of the warm climatesand sluggish circulation of the Cretaceous, but also be-cause of the unique circulation history of the Basin andthe extent of the topographic barriers between it and theadjacent, better-oxidized Pacific Ocean. Today's analogfor the North Atlantic in stages B and D (Fig. 9) is theBlack Sea, with the Mediterranean instead of the Pacificas a nearby source of deep water. The change in state ofthe North Atlantic from C (oxidizing) to D (reducing) inFigure 9 mimics the change in circulation of the BlackSea as the sea level rose during the Holocene transgres-sion (see Degens and Mopper, 1976). Both the Black Seaand the Early Cretaceous North Atlantic are silledbasins in which the deposition of organic matter de-pended not on the fact that the basins were silled, but ontheir circulation history—as in other basins (Demaisonand Moore, 1980). Bottom water in the Black Sea onlybecame reducing when the Bosporus sill was deepenough to permit substantial inflow of Mediterraneandeep water during the Holocene rise in sea level (Degensand Mopper, 1976).

The final stage of the evolution of the North Atlantic(Fig. 9F) involved its widening to the point where a deepcirculation system vigorous enough to keep the bottomoxygenated could develop; it is not clear if Pacific in-flow continued through this period, though it seemsprobable that it did.

During the Early and mid-Cretaceous the influx ofPacific intermediate water rich in nutrients would haveled to an increase in the vertical nutrient gradient, thus,eventually, to a slow increase in the productivity of sur-

face waters (through stages B and D in Fig. 9). Thiswould explain the increase in both peak and backgroundTOC flux from Subunits 5c through 4d and from Sub-unit 4c into Subunit 4b (Fig. 4B, 4C) as well as theValanginian-Barremian TOC rise shown in Figure 6. Italso explains why the Barremian-Aptian has a moremarine organic facies than the Valanginian-Hauteri-vian, and may explain why the Cenomanian has a moremarine organic facies than the Albian.

The development of a Mediterranean type of circula-tion in stage C (Fig. 9) would have led to depletion ofnutrients in surface waters and to a low TOC flux in theAptian-Albian (Subunit 4c—Fig. 4B, 4C). The organicfacies would be less marine, exposing the terrestrial con-tribution.

We do not see in the deep western North Atlanticstrong evidence for a late Barremian-Aptian "oceanicanoxic event" of the type described by Schlanger andJenkyns (1976) and Jenkyns (1980), although there isevidence for their Cenomanian "oceanic anoxic event"(Summerhayes, 1981). Our data suggest that the deposi-tion of organic matter in the Early Cretaceous NorthAtlantic Basin was not confined in time to a late Bar-remian-Aptian event (Figs. 4 and 6). If the event is realelsewhere in the world ocean, this lack of correspon-dence reflects the fact that the more or less enclosedNorth Atlantic Basin has a history independent of thatof other nearby basins (the Pacific, for instance). Thecorrespondence between the history of burial of organicmatter in the deep North Atlantic (Figs. 4 and 6) and theδ13C shift in the Valanginian-Aptian (Fig. 8) suggeststhat processes in the North Atlantic, rather than world-wide "oceanic anoxic events," influenced the isotopiccharacter of oceanic surface water of this Basin in theEarly and middle Cretaceous.

CONCLUSIONS

Plate tectonics, paleoclimate, and eustatic changes insea level strongly influence the organic facies of Meso-zoic sediments from the deep western North Atlantic. Inthe Jurassic, when the North Atlantic was narrow andnot connected at depth to the Pacific, bottom waterswere oxidizing except for short periods of time, as in theCallovian, when they became anoxic. Organic-rich sedi-ments from periods of anoxic bottom water have a dom-inantly terrestrial organic facies, suggesting low produc-tivity.

In the Early Cretaceous, following a drop in sea levelat the end of the Berriasian, a connection to the Pacificwas established, permitting the influx of nutrient-rich,oxygen-poor subsurface waters. Bottom water becamemore or less permanently anoxic. Terrigenous influxwas high near the coast (Site 534), diluting marine or-ganic matter and supplying lesser amounts of terrestrialorganic matter in its place. This influx diminished notonly away from shore, but also—as sea level rose—through time. The vertical nutrients gradient built upover the same period causing surface waters to becomemore productive. As a result, TOCs are highest, and theinfluence of marine organic matter is strongest, in theBarremian to early Aptian.

479


A sea-level drop in the late Aptian cut off the influxof Pacific intermediate water. Circulation became "Med-iterranean" (with a negative instead of a positive waterbalance) and oxidizing. Nutrient levels and productivitydropped. Terrigenous influx increased. The low supplyof marine organic matter and the poor preservation ofboth marine and terrestrial organic materials make thisperiod organic-poor.

Later, during the Albian rise in sea level, the Pacificconnection was reestablished, bottom waters becameagain predominantly reducing, and surface water nutri-ents and productivity increased. The rate of accumula-tion of elastics dropped, as did the influx of terrestrialorganic matter.

A further change in circulation, caused by the open-ing of a connection to the South Atlantic, may havetaken place in the Cenomanian. It is not obvious in thesedimentary record from Sites 391 or 534 (in the datathat we have examined), which leads us to suggest thatits effects may have been removed by erosion from theBlake-Bahama Basin.

ACKNOWLEDGMENTS

We thank Exxon Production Research Company for permission topublish this work. R. Cunningham, P. M. Kroopnick, E-Chien Foo,and D. Gilbert provided helpful comments on the ideas presentedhere. TOCs were analyzed by M. S. Bisotooni of EPRCo, Rock-Evalpyrolysis was carried out by K. Hahn of EPRCo, and elementalanalyses were performed by Robertson Research Inc., of Houston. T.C. Masran analyzed the organic matter types by transmitted-lightmicroscopy.

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Date of Initial Receipt: April 19, 1982

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